Conventionally, a waste heat recovery Rankine cycle system is known in which waste heat radiated from a heat source such as an engine is collected so as to generate steam with high temperature and high pressure, and an expander is driven with the steam as a working medium so as to generate electric power. Among such waste heat recovery Rankine cycle systems, there is a system configured by combining a first Rankine cycle operated with water vapor (water) as a working medium and a second Rankine cycle operated with a material having lower boiling point than the water as a working medium. The steam of the second working medium is generated with the water vapor which is the first working medium as the heat source, and the electric power is generated again with the steam. By reusing the first working medium as the heat source heating the second working medium, efficiency of electric power generation is improved. For example, the art described in the Patent Literature 1 is so.